The invention is in the field of well logging, in which measurements taken in a borehole are used in searching for and exploiting valuable underground resources such as oil and gas. It is particularly directed to a method and a system for natural gamma radiation logging, in which a log is derived of the radiation detected in selected energy windows and is converted into a log of selected subsurface materials, such as thorium, uranium and potassium. Yet more specifically, the invention is directed to deriving a log of the selected subsurface materials which is substantially corrected for errors due to factors such as radiation emitting materials (e.g. potassium, in the form of potassium chloride) in the borehole fluid, and radiation absorbing materials, such as barite and/or hematite, in the same mud filtrate.
In prior art natural gamma radiation logging, a tool capable of detecting gamma radiation in each respective one of several (e.g. five) energy windows is passed through a selected borehole interval, and a record is made of the gamma ray photons detected within the respective windows. The gamma rays are emitted in the decay of subsurface material such as thorium, uranium and potassium (Th,U,K), each of which emits rays with characteristic energy spectra. The tool output is converted to a log of the concentrations of Th,U,K at the respective borehole depth levels.
The Th,U,K log is important in searching for and exploiting underground resources because it is believed that these materials appear in nature with a discernible relationship to geology and rock morphology. This log is particularly useful in the exploration for and exploitation of oil and gas resources because it is believed that the concentrations of Th,U,K taken individually or in combination are a good indication of previously unavailable information as to the presence, type and volume of shale or clay in the formations surrounding the borehole.
While the fact that Th, U and K emit characteristic and discrete energy spectra should in principle allow the detected radiation to be separated by source material, in practice the nature of the logging process makes the detected spectra continuous, with poor energy resolution and poor counting statistics. Nevertheless, there are known techniques for usefully estimating and logging the Th,U,K concentrations.
The difficult measurement conditions in Th,U,K logging have been made yet more difficult in recent times by the more common use of borehole fluids (drilling mud) containing potassium chloride (KCl) and weighting materials which are strong absorbers of gamma rays, such as barite and/or hematite (B). These borehole fluids stabilize the borehole by reducing clay and shale hydration and provide various other benefits. However, the KCl in the borehole fluid emits its own gamma radiation whose contribution is merged with that of potassium in the undisturbed formations surrounding the borehole, while the strong absorber (B) in the same borehole fluid can significantly reduce the gamma radiation flux from the surrounding formations. Typically, the drilling and logging environment makes it impossible or impractical to measure the concentration of KCl and B in the mud at the time the borehole logging commences, and no prior art technique is known which satisfactorily corrects for their effect on the Th,U,K log. Accordingly, a major aspect of the invention is directed to a process and a system for substantially overcoming the deleterious effect of this type of borehole fluid in natural gamma radiation logging used to find the concentrations of materials such as thorium, uranium and potassium in the undisturbed formations around the borehole.
It is known that barite in the mud has a significant effect in nuclear (scattered gamma radiation) logging. See Seeman U.S. Pat. No. 3,900,733 and references cited therein, for a discussion of techniques attempting to correct for the barite effect. It is also known that KCl in the mud filtrate has a significant effect in natural gamma radiation logging. See Cox, J. W. et al., "The Effect Of Potassium-Salt Muds On Gamma-Ray, And Spontaneous Potential Measurements," SPWLA 1976, and references cited therein. It is believed, however, that neither this nor other similar known prior art teaches techniques for satisfactorily correcting for these deleterious effects of the borehole fluid in natural gamma radiation logging.
In general terms, the invention comprises deriving a log of gamma radiation detected in selected energy windows, e.g., five, for a selected borehole interval, and converting it into a log of the selected materials, e.g. Th,U,K, which is substantially corrected for at least one of: (i) a gamma ray emitter in the borehole fluid, e.g., potassium salts and (ii) a gamma ray attenuator in the borehole fluid, e.g., a strong attenuator (B) such as barite and/or hematin. The invention makes use of the recognition that the availability of more than three (e.g., five) energy windows and the way Th, U and K contribute thereto can be used to find not only the three unknown concentrations Th,U,K but also at least one, and preferably both of two other factors: the KCl and B in the borehole fluid. More specifically, the invention makes use of the recognition that the potassium gamma rays contribute only or primarily to the three lowest energy windows and that the spectrum of gamma rays from the potassium in the borehole fluid is somewhat different from that in the undisturbed formations surrounding the borehole. The invention makes use of the additional recognition that the unscattered thorium gamma rays are detected mostly in window 5 and much less in window 4 while the unscattered uranium gamma rays are detected to a substantial extent in each of windows 4 and 5, and that the additional response in the lower windows for thorium and uranium is due primarily to interaction of the thorium and uranium gamma rays in the undisturbed formations, the borehole and the tool itself. The invention further makes use of the discovery that if the concentrations of thorium and uranium are estimated based only on the basis of the two highest energy windows, 4 and 5, these estimates should agree with the thorium and uranium is due primarily to interaction of the thorium and estimates based on all five windows if there is no KCl in the mud filtrate, and that a disagreement which cannot be attributed to statistical fluctuations can be used as a measure of the presence of mud filtrate KCl. The invention yet further makes use of the recognition that the strong absorber in the mud filtrate reduces the flux of low energy gamma rays reaching the tool, depresses the uranium estimate based on all five windows and increases the thorium estimate, and that the thorium and uranium estimates have about the same sensitivity to the strong absorber.
In a particular and nonlimiting example of an embodiment of the invention, the concentrations of the three materials (Th,U,K) are related through an empirically derived logging tool sensitivity matrix to five corrected window measurements. The first corrected window measurement is the output of the logging tool for window 1 modified by the unknown KCl concentration and B correction, and also modified for the caliper (diameter) of the borehole. The second and third corrected window measurements are the respective outputs of the second and third tool windows, respectively modified by the unknown KCl concentrations and by the borehole caliper. The fourth and fifth "corrected" window measurements are simply the outputs of the logging tool for the fourth and fifth windows. This can be done for each depth level in the selected borehole depth interval but, in order to reduce processing time, the tool window outputs can be averaged over several depth levels, particularly in intervals where the borehole does not change greatly, such that the depth interval for a sample of the energy windows log is, e.g., four feet rather than six inches. The borehole fluid potassium concentration (KCl) and strong absorber correction (B) are then found for each of the depth levels of interest.
The KCl concentrations and B corrections found as described above for the individual depth levels in the borehole typically differ significantly from one depth level to another. However, drilling and logging experience suggests that the KCl is typically well mixed in the borehole fluid, and that its concentration should be reasonably constant throughout the well. In the case of the strong absorber, experience suggests that its effective absorption should be mainly related to the caliper of the borehole and should not be significantly perturbed by variable mudcake build-up. A simple average of the KCl concentrations and B corrections found at the respective depth levels may in a given case be a close estimate of the true KCl and B content of the borehole fluid, but this need not be true in all cases because the scatter in the KCl and B estimates is due to many factors which could not be said to be linear or consistent from one depth to another or from one borehole to another. Hence, in accordance with another aspect of the invention, the simple averages of the KCl concentrations and B corrections are used only as the starting estimates in a process which finds the best fit of a KCl concentration and a B correction to the thorium and uranium estimates throughout the borehole depth interval which is of interest.